Air foil bearing

ABSTRACT

Air foil bearing capable of adjusting stiffness of single bumps by varying widths of the single bumps. Since the bumps have the same height in an initial stage, a wedging effect is implemented only when a load is applied thereto by an oil film. Therefore, the air foil bearing is advantageous in terms of height management thereof. In addition, it is possible to stably dampen shocks applied in the direction of rotation of a rotor, to improve a support capability, and to reduce shocks due to damping. Therefore, it is possible to enhance durability of the air foil bearing.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a 371 of International Application No.PCT/KR2016/013329 filed Nov. 18, 2016, which claims priority from KoreanPatent Application Nos. 10-2015-0161838 filed on Nov. 18, 2015,10-2015-0166595 filed on Nov. 26, 2015, 10-2016-0034023 filed on Mar.22, 2016 and 10-2016-0044117 filed on Apr. 11, 2016.

BACKGROUND OF THE INVENTION Field of the Invention

Exemplary embodiments of the present invention relate to an air foilbearing, and more particularly, to an air foil bearing capable ofimplementing stiffness control and a wedging effect.

Description of the Related Art

A bearing is a mechanical element that fixes a rotary shaft at a certainposition and rotatably supports the shaft while supporting a self-loadof the shaft and a load applied to the shaft. A ball bearing or ajournal bearing is a bearing that supports a shaft using an oil film,and a foil bearing is a bearing that supports a shaft using ahigh-pressure air layer formed between a top foil and the shaft.

An air foil bearing supports an axial load by forming a pressure withair, as a viscous fluid, introduced between foils coming into contactwith a rotor or a bearing disc when the rotor rotates at high speed.

Since the air foil bearing is effective to support a rotary shaftrotating at high speed, it is applicable to a rotary shaft rotating athigh speed in a rotating machine such as a turbo compressor, a turbocooler, a turbo generator, or an air compressor.

Korean Patent No. 1396889 discloses an example of such an air foilbearing.

This air foil bearing typically has a structure in which a bump foil anda top foil are disposed between a pair of disc-shaped plates. Since theload support capability of the air foil bearing is determined by thetotal pressure of air formed in the bearing, there is a need to increasethe total pressure. However, it is difficult for a conventional air foilbearing to improve a load support capability because it does not have astructure to increase the pressure of air.

RELATED ART DOCUMENT Patent Document

(Patent Document) Korean Patent No. 1396889 (May 13, 2015)

SUMMARY OF THE INVENTION

An object of the present invention is to provide an air foil bearingcapable of enhancing damping efficiency.

Other objects and advantages of the present invention can be understoodby the following description, and become apparent with reference to theembodiments of the present invention. Also, it is obvious to thoseskilled in the art to which the present invention pertains that theobjects and advantages of the present invention can be realized by themeans as claimed and combinations thereof.

In accordance with one aspect of the present invention, an air foilbearing is an air foil bearing for rotatably supporting a rotary shaft.The air foil bearing includes a disc-shaped plate, a plurality of bumpfoils coupled to the plate, and a top foil disposed above each of thebump foils while one end of the top foil is coupled to the plate and theother end thereof is a free end.

Each of the bump foils may include a plurality of single bumps spacedapart from each other in a plurality of rows, and the single bumps mayhave the same stiffness or stiffness gradually increased from an innerdiameter side of the plate to an outer diameter side thereof.

Widths (A to D) of respective rows of the single bumps may be graduallyincreased toward the outer diameter side of the plate, or the singlebumps may have larger widths (A to D) at the outer diameter side of theplate rather than at the inner diameter side thereof.

The single bumps may have lengths of bump ridges (L) gradually increasedfrom the inner diameter side of the plate to the outer diameter sidethereof.

The single bumps disposed at the inner and outer diameter sides of theplate may have longer arc lengths than the single bumps disposed inward.

Each of the bump foils may include first to fifth single bumps obliquelyarranged radially of the plate, the first to fifth single bumps may formfirst to fourth rows and are spaced apart from each other, and thesingle bumps may have the same stiffness or stiffness graduallyincreased from an inner diameter side of the plate to an outer diameterside thereof.

Widths (A to D) of the first to fourth rows of the single bumps may begradually increased toward the outer diameter side of the plate.

Widths (C and D) of the third and fourth rows of the single bumps may belarger than widths (A and B) of the first and second rows of the singlebumps.

The single bumps may have lengths of bump ridges (L) gradually increasedfrom the inner diameter side of the plate to the outer diameter sidethereof.

Arc lengths of the first and fourth rows of the single bumps may belonger than arc lengths of the second and third rows of the singlebumps.

In accordance with another aspect of the present invention, an air foilbearing is an air foil bearing for rotatably supporting a rotary shaft.The air foil bearing includes a disc-shaped plate, a plurality of bumpfoils coupled to the plate, and a top foil disposed above each of thebump foils while one end of the top foil is coupled to the plate and theother end thereof is a free end. Each of the bump foils includes a firstfoil, and a second foil arranged within the first foil.

The first and second foils may be spaced apart from each other with aslit interposed therebetween.

The first foil may have a higher stiffness than the second foil.

The first and second foils may be made of different materials.

The first and second foils may be made of the same material.

The first foil may have a larger thickness than the second foil.

The first foil may have a smaller bump width than the second foil.

The second foil may have a slit formed therein to bisect the widththereof, and the width of the slit between the first foil and the secondfoil may correspond to the width of the slit of the second foil.

Alternatively, the second foil may have a slit formed therein to bisectthe width thereof, and the width of the slit between the first foil andthe second foil may be larger than the width of the slit of the secondfoil.

The first and second foils may consist of a plurality of foils, and thefirst and second foils may be disposed with a second slit interposedtherebetween, the second slit having different widths and shapes.

The first slit between the first foil and the second foil may have thesame width from one end thereof from the other end thereof.

The second slit between the second foils may have a width increased in adirection opposite to a trailing edge (F) connecting the first andsecond foils from the trailing edge (F) in an end of the second slit inthe direction of rotation of the rotor.

Alternatively, the second slit between the second foils may have astepped width increased in the direction opposite to the trailing edge(F).

The second foil may have a smaller width at an end thereof opposite tothe trailing edge (F) than an end thereof toward the trailing edge (F).

The second foil may include at least one stepped portion cut in astepped form at facing one side thereof, and a connection portionextending in the direction of rotation of the rotor to connect thestepped portion.

An end of the first foil opposite to the trailing edge (F) is coupled byspot welding.

The first foil may have a smaller bump height than the second foil.

A distance (D) from an end of the slit toward the trailing edge (F) tothe trailing edge (F) may be less than or equal to twice a bump pitch(C).

In accordance with a further aspect of the present invention, an airfoil bearing is an air foil bearing for rotatably supporting a rotaryshaft. The air foil bearing includes a disc-shaped plate, a plurality ofbump foils coupled to the plate, and a top foil disposed above each ofthe bump foils while one end of the top foil is coupled to the plate andthe other end thereof is a free end, wherein each of the bump foilsincludes first to fourth foils extending in a direction opposite to adirection of rotation of a rotor.

Ends of the first to fourth foils corresponding to the direction ofrotation of the rotor may be connected as one piece.

A leading ridge (A) of each of the first to fourth foils, which isadjacent to an end thereof in a direction opposite to the direction ofrotation of the rotor, may have a height different from another adjacentbump ridge.

The height of the leading ridge (A) of each of the first to fourth foilsmay be lower than the height of another adjacent bump ridge.

The height of the leading ridge (A) of each of the first to fourth foilsmay be set to be within 70% of the height of another adjacent bumpridge.

The top foil may have an inclined section (L1) in which a gap betweenthe top foil and the plate is gradually increased, and a flat section(L2) in which the gap therebetween is constant.

The leading ridge (A) of each of the first to fourth foils maycorrespond to a starting position of the flat section (L2) of the topfoil.

An end (B) adjacent to the leading ridge (A) of each of the first tofourth foils may be coupled to the plate by spot welding.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional view illustrating an installation example ofan air foil bearing according to embodiments of the present invention;

FIG. 2 is a top view illustrating an air foil bearing according to afirst embodiment of the present invention;

FIG. 3 is a view schematically illustrating a bump foil of FIG. 2.

FIG. 4 is a top view illustrating an air foil bearing according to asecond embodiment of the present invention;

FIG. 5 is a top view illustrating an air foil bearing according to athird embodiment of the present invention;

FIGS. 6 and 7 are top views illustrating a welded portion of an air foilbearing according to different embodiments of the present invention;

FIG. 8 is a graph illustrating a pressure distribution according to theair foil bearing of FIG. 6;

FIG. 9 is a top view illustrating an air foil bearing according to afourth embodiment of the present invention;

FIG. 10 is a top view illustrating an air foil bearing according to afifth embodiment of the present invention; and

FIG. 11 is a top view illustrating an air foil bearing according to asixth embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to exemplary embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The present invention may, however, be embodied in differentforms and should not be construed as limited to the embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey thescope of the present invention to those skilled in the art. Throughoutthe disclosure, like reference numerals refer to like parts throughoutthe various figures and embodiments of the present invention.

Hereinafter, an air foil bearing according to an embodiment of thepresent invention will be described in detail with reference to theaccompanying drawings.

FIG. 1 is a cross-sectional view illustrating an installation example ofan air foil bearing according to an embodiment of the present invention.

As illustrated in FIG. 1, the air foil bearing according to theembodiment of the present invention is installed in a machine having arotary shaft rotating at high speed. Herein, an example, in which an airfoil bearing 700 is installed to support a rotary shaft 650 of a blowermotor 600 mounted in an air compressor 10, will be described forconvenience (however, this is merely by way of example, and it may beapplied to any machine having a rotating shaft).

The air compressor 10 for a vehicle includes a housing 100 that definesthe external appearance thereof, an impeller 400 that is coupled to thefront of the housing 100 to compress air, an impeller receiving part 200and an impeller housing 300 for receiving the impeller 400, a rear cover500 that is coupled to the rear of the housing 100, and a blower motor600 that is installed in the housing 100 to rotatably drive the impeller400.

The impeller housing 300 has an air inlet 310 formed at the center ofthe front thereof for introduction of outside air, and air outlets 330formed at both sides of the front thereof. The impeller 400 is installedin the impeller housing 300, and a rotary shaft 650 of the blower motor600, which will be described later, is coupled through the hollow of theimpeller 400. That is, the impeller 400 is supported by the rotary shaft650. The air introduced through the air inlet 310 by the impeller 400 iscompressed by the impeller 400 and is then discharged to the air outlets330.

The blower motor 600 is inserted into a motor housing 600 a insertedinto the housing 100. The blower motor 600 includes a stator 630 that isinstalled adjacent to the inner peripheral surface of the motor housing600 a and has a hollow (not shown), a rotary shaft 650 that is installedthrough the hollow of the stator 630, and a rotor 610 that is coupled tothe outer peripheral surface of the rotary shaft 650.

The rotary shaft 650 is rotatably supported by a thrust bearing 700 anda journal bearing 750, which are installed behind the impeller 400,within the housing 100 in a state in which one end of the rotary shaft650 is coupled through the hollow of the impeller 400, and the rear endof the rotary shaft 650 is also rotatably supported by a rear bearing800.

Hereinafter, the air foil bearing according to an embodiment of thepresent invention will be described in more detail with reference to theaccompanying drawings.

FIG. 2 is a top view illustrating the air foil bearing according to theembodiment of the present invention. FIG. 3 is a view schematicallyillustrating a bump foil of FIG. 2.

A rotor disc (not shown) having a disc shape is formed in front of therotary shaft 650, and the thrust bearing 700 is inserted adjacent to onesurface in front of and the other surface behind the rotor disc (seeFIG. 1).

As illustrated in FIGS. 2 and 3, the thrust bearing 700 according to theembodiment of the present invention is an air foil bearing, and has astructure in which a plurality of bump foils 706 having a fan shape areseated to a disc-shaped plate 702 and each is covered with a top foil704. The thrust bearing 700 has a circular hole formed in the centerthereof for insertion of the rotary shaft 650. In a state in which therotary shaft 650 is inserted into the hole of the thrust bearing 700,one surface of each of the bump foils 706 is adjacent to one surface ofthe rotor disc and the other surface thereof is seated to the plate 702.

One end of the top foil 704 is fixed to the plate 702 and the other endthereof is spaced from the plate 702 to be a deformable free end. Thebump foil 706 is a fan-shaped plate, and is in contact with the top foil704 between the fixed end and the free end of the top foil 704. The topfoil 704 and the bump foil 706 are attached to the plate 702 by welding.

Air is introduced between air foils in the rotor disc along with thehigh-speed rotation of the rotor 630 to form a pressure therebetween,thereby supporting a load during the rotation of the rotor. In thiscase, the air has characteristics of a viscous fluid. That is, thisexpression means that air does not have viscosity in practice, but ithas viscosity when supporting a load by introduction of air between airfoils.

In thrust bearings 700 having the same pitch, height, length, andthickness, a load support capability is affected by the stiffness of thebump foil 706 as well as the viscosity of air.

The stiffness of each of the bump foils 706 is determined by the area ofa bump projected on the plane. It is preferable that the stiffness ofthe bump foil 706 be uniform in all sides of the plate 702 or begradually increased from the inner diameter side of the plate 702 to theouter diameter side thereof (from row 1 to row 4). The reason isbecause, when the rotor rotates, a load is concentrated in the directionof row 4 to thereby cause severe abrasion. Accordingly, it isadvantageous in terms of durability for the stiffness of the bump foil706 to increase in the direction of row 4.

However, a conventional bump foil is configured such that the rowsthereof have the same width (see A, B, C, and D in FIG. 3). In thiscase, single bumps (see numbers 1 to 5 of FIG. 3, they being designatedin sequential order by numbers 1 to 5 in the direction of rotation of arotor) have different stiffnesses. Especially, it is verydisadvantageous in terms of uniform stiffness distribution since thestiffness at the outer diameter side of each bump is lower than that atthe inner diameter side of the bump on the basis of the direction ofrotation of the rotor. Accordingly, if the rows of the single bumps havethe same width (see A, B, C, and D in FIG. 3) as in the related art, itis impossible for the stiffness of the bump foil to be uniform or begradually increased in the direction in which a load is increased.

Accordingly, to resolve this problem in the present invention, diagonallengths (bump ridges, which are convexly formed when viewing the singlebumps from the side, they being designated by reference numeral L) ofsingle bumps (1 to 5) have to be adjusted, and the diagonal lengths (L)may be changed by adjusting widths (A to D) of respective rows of thesingle bumps (1 to 5) (in this case, the single bumps are designed tohave the same bump ridge height).

When, in the widths (A to D) of the respective rows of the single bumps(1 to 5), the respective widths of rows 1 to 4 are set as A, B, C, andD, A may be equal to B, C may be larger than A and B, and D may belarger than C (A=B<C<D).

When the single bumps have the widths described above, the lengths ofthe bump ridges (L) are increased from owl to row 4 and the areas of thesingle bumps are also increased, thereby increasing a load supportcapability.

In addition, it is possible to implement a wedging effect of preventinga leak of air forming an oil film by adjusting the lengths of the singlebumps (1 to 5) (arc lengths of rows 1 to 4 in the direction of rotationof the rotor).

That is, by forming the lengths of the single bumps (1 to 5) such thatthe rows 2 and 3 are tensioned inward of the rows 1 and 4, the area ofan air pocket (P) for capturing air between the top foil 704 and thebump foil 706 is increased. Thus, it is possible to prevent a loss ofthe oil film by the wedging effect. Here, the wedging effect means adynamic pressure (pressure) is generated while a fluid is pulled by theviscosity thereof when the space storing the fluid is narrowed in awedging manner, in which case the fluid is pushed into a wedge gap, sothat a load is supported by the pressure. In addition, a damping forceis improved as the area of the air pocket (P) is increased, with theconsequence that the non-contact area of the top foil 704 is increased.Therefore, it is possible to reduce friction applied to the top foil704.

According to the above-mentioned configuration, since the bump foils 706have the same bump ridge height in an initial stage, the wedging effectis implemented only when a load is applied thereto by the oil film.Therefore, the thrust bearing 700 is advantageous in terms of heightmanagement thereof.

In addition, in the case where the single bumps have the same shape, thestiffness of the single bumps is increased when the widths of the singlebumps are increased. Thus, it is possible to adjust the stiffness of thesingle bumps by varying the widths of the single bumps under thecondition that the single bumps have the same shape.

As is apparent from the above description, an air foil bearing accordingto exemplary embodiments of the present invention can adjust stiffnessof single bumps by varying widths of the single bumps. In addition,since the bumps have the same height in an initial stage, a wedgingeffect is implemented only when a load is applied thereto by an oilfilm. Therefore, the air foil bearing is advantageous in terms of heightmanagement thereof.

FIG. 4 is a top view illustrating an air foil bearing according to asecond embodiment of the present invention. FIG. 5 is a top viewillustrating an air foil bearing according to a third embodiment of thepresent invention.

As illustrated in FIG. 4, a thrust bearing 700 according to a secondembodiment of the present invention is an air foil bearing, and has astructure in which a plurality of bump foils 706 having a fan shape areseated to a disc-shaped plate 702 and each is covered with a top foil704. The thrust bearing 700 has a circular hole formed in the centerthereof for insertion of a rotary shaft 650. In a state in which therotary shaft 650 is inserted into the hole of the thrust bearing 700,one surface of each of the bump foils 706 is adjacent to one surface ofa thrust disc 652 and the other surface thereof is seated to the plate702.

One end of the top foil 704 is fixed to the plate 702 and the other endthereof is spaced from the plate 702 to be a deformable free end. Thefixed end of the top foil 704 is attached to the plate 702 by welding.

The bump foil 706 is a fan-shaped plate, and is in contact with the topfoil 704 between the fixed end and the free end of the top foil 704. Thebump foil 706 consists of two foils, which are a “⊏”-shaped foil formingthe edge of the fan-shaped bump foil and a foil arranged within the“⊏”-shaped foil.

In more detail, the bump foil 706 includes a connection foil 706 a thatforms the radial edge of the fan-shaped bump foil, a first foil 706 bthat forms the outer diameter edge of the fan-shaped bump foil, a secondfoil 706 c that forms the inner diameter edge of the fan-shaped bumpfoil, and third and fourth foils 706 d and 706 e that are spaced betweenthe first foil 706 b and the second foil 706 c with slits C1 to C3interposed therebetween. The free end of each of the third and fourthfoils 706 d and 706 e is also spaced apart from the connection foil 706a with a slit C4 interposed therebetween. A portion of the edge of thethird foil 706 d is diagonally cut, and the portion between the firstfoil 706 b and the connection foil 706 a is diagonally cut correspondingto the diagonally-cut portion. The diagonally-cut portions are spacedapart from each other with a cut slit 704 a interposed therebetween. Theends of the first to fourth foils 706 b, 706 c, 706 d, and 706 e arewelded to the plate 702 (wherein, their welded portions will bedescribed later), and the other ends thereof extend in an arc directionof the fan-shaped bump foil.

The lengths of the first to fourth foils 706 b, 706 c, 706 d, and 706 eare shortened from the first foil 706 b to the fourth foil 706 e. Thethird and fourth foils 706 d and 706 e are disposed at the side that isless vulnerable to the pressure generated during rotation of the rotor,compared to the first and second foils 706 b and 706 c. Therefore, thethird and fourth foils 706 d and 706 e are preferably made of a materialhaving lower stiffness than the first and second foils 706 b and 706 c.Although the first to fourth foils 706 b, 706 c, 706 d, and 706 e aremade of the same material, the third and fourth foils 706 d and 706 emay have lower stiffness than the first and second foils 706 b and 706 cin such a way to widen the width of the bump forming the third andfourth foils 706 d and 706 e and to increase an amount of deformation ofthe bump forming the third and fourth foils 706 d and 706 e than that ofthe bump forming the first and second foils 706 b and 706 c.

Since the amount of deformation of the third and fourth foils 706 d and706 e is increased when the third and fourth foils 706 d and 706 e havelower stiffness than the first and second foils 706 b and 706 c, it ispossible to capture a large amount of air to thereby increase a loadsupport capability.

Meanwhile, the above slits consist of a first slit C1, a second slit C2,a third slit C3, and a fourth slit C4. The first to third slits C1 to C3serve to maintain a minimum gap such that the first to fourth foils 706b, 706 c, 706 d, and 706 e do not come into contact with one another.

Preferably, the fourth slit C4 has a gap such that the third and fourthfoils 706 d and 706 e do not come into contact with the connection foil706 a even though the bump ridges of the third and fourth foils 706 dand 706 e are pressed so that the third and fourth foils 706 d and 706 eare tensioned when the bump foil 706 is deformed. In addition, fluidsare introduced in an axial direction and a direction of rotation of therotor when the rotor rotates. In this case, it is possible to form aspace for stable movement of the fluids when the fluids aresimultaneously or randomly introduced into the fourth slit C4 and thefirst to third slits C1 to C3, thereby enhance a stable dampingcapability.

All of the first to third slits C1 to C3 are formed in thecircumferential direction of the plate 702, and the fourth slit C4 isradially formed. Therefore, it is possible to stably dampen vibrationdue to introduction and discharge of fluid.

As illustrated in FIG. 5, a thrust bearing 700′ according to a thirdembodiment of the present invention has the same configuration as thataccording to the above second embodiment, but first to third slits C1 toC3 according to the third embodiment are set to have a gap differentfrom those according the second embodiment.

That is, the first to third slits C1 to C3 may be designed to have thesame gap as a fourth slit C4.

In this case, third and fourth foils 706 d′ and 706 e′ have a relativelysmaller width than first and second foils 706 b′ and 706 c′. Thisenables the third and fourth foils 706 d′ and 706 e′ to perform dampingdifferent from the first and second foils 706 b′ and 706 c′. Throughsuch a configuration, the axial stress applied to the rotor is stablysupported and distributed to prevent damage of the bearing.

Since the first to third slits C1 to C3 have the same gap as the fourthslit C4, the thrust bearing 700′ according to the third embodiment ofthe present invention has a damping capability and a load supportcapability different from that according to the second embodiment.

Thus, since an amount of damping varies and amounts of deformation ofthe third and fourth foils 706 d′ and 706 e′ are increased, it ispossible to minimize stress concentration and a load in the axialdirection due to introduction and movement of fluid.

Therefore, it is possible to minimize vibration applied to the rotor andthus to enhance an efficiency for vibration damping. In addition, thebearing can be stably used for a long time by minimizing vibration andshocks applied to a compressor or an air blower and simultaneouslyenhancing durability and capability for vibration damping.

The welded portion of the air foil bearing according to the embodimentsof the present invention having the above configuration will bedescribed in detail.

FIGS. 6 and 7 are top views illustrating a welded portion of an air foilbearing according to different embodiments of the present invention.

As illustrated in FIGS. 6 and 7, a bump foil 706 or 706′ consists of afirst foil 706 a or 706 a′ that forms the edge of the fan-shaped bumpfoil and has a substantially “⊏” shape, and a second foil 706 b or 706b′ that is arranged within the first foil 706 a or 706 a′ and is spacedapart from the first foil 706 a or 706 a′ with a slit 706 c or 706 c′interposed therebetween. The ends of the first foil 706 a or 706 a′ andsecond foil 706 b or 706 b′ may be welded to the plate 702 and the otherends thereof may extend in an arc direction of the fan-shaped bump foil.Since the air foil bearings illustrated in FIGS. 6 and 7 are designatedby different reference numerals from but have the same structure asthose of the embodiments illustrated in FIGS. 4 and 5, a detaileddescription thereof will be omitted.

The welded portion of FIG. 6 or 7 (corresponding to reference numeral706 f or 706 f′ of FIG. 4 or 5) is disposed in the direction of rotationof the rotor. The welded portion has a relatively smaller width,compared to the first foil 706 a or 706 a′ and the second foil 706 b or706 b′, and maintains only a fixed state. In addition, since the weldedportion is not formed with an uneven portion such as a bump ridge, thewelded portion does not perform damping according to introduction anddischarge of fluid but stably maintains a fixed state.

FIG. 8 is a graph illustrating a pressure distribution according to theair foil bearing of FIG. 6.

As illustrated in FIG. 8, when the bump foil 706 has uniform stiffness,the pressure of the bump foil 706 in the radial direction of the plate702 (wherein, the center of the plate is defined as d1 and the outsidethereof is defined as d2 at any point of FIG. 2) is set as P1. When thebump foil includes the first and second foils 706 a and 706 b, thepressure of the bump foil 706 in the radial direction of the plate 702is set as P2. It can be seen that the pressure is concentrated betweenthe radial inside d1 and the radial outside d2 in the case of P1,whereas the pressure relatively evenly distributed between the radialinside d1 and the radial outside d2 in the case of P2.

If the pressure is concentrated on one portion, the bump foil 706frequently comes into contact with the top foil 704 in the aboveportion, which may lead to an increase in abrasion of the top foil 704.This also means that the edge of the bump foil 706 close to the radialinside d1 and outside d2 is not sufficiently pressed due to a low loadsupport capability.

However, when the stiffness of the first foil 706 a is set to be higherthan the stiffness of the second foil 706 b as in the embodiments of thepresent invention, the load support capability is enhanced with theconsequence that the radial inside d1 and outside d2 may be sufficientlypressed. Thus, the pressure P2 is not concentrated on one portion but itis evenly distributed.

The air foil bearing according to the embodiments of the presentinvention having the above configuration can enhance a load supportcapability and increase a damping effect by changing the shape andstiffness of the bump foil. In addition, it is possible to enhance thedurability of the bearing by an improvement in damping capability.

In the air foil bearing according to the embodiments of the presentinvention, it is possible to prevent the end of the bump foil fromsharply protruding when the bump foil is deformed by changing the shapeof the bump foil and the shape of the slit. Hereinafter, an embodimentin which shapes of a bump foil and a slit are changed will be describedin detail, and the change of these shapes may be applied to the aboveembodiments. A detailed description of the same configuration as theabove embodiments will be omitted. For convenience, the outer arc of thefan-shaped bump foil is defined as an outer diameter direction and theinner arc thereof is defined as an inner diameter direction in FIG. 9.

FIG. 9 is a top view illustrating an air foil bearing according to afourth embodiment of the present invention.

As illustrated in FIG. 9, the air foil bearing includes a bump foil 706″arranged between a top fail 704″ and a plate 702″.

The bump foil 706″ consists of first foils 706 a″ that are disposed atthe outer and inner diameter sides of the top foil in the direction ofrotation of a rotor (in the arrow direction of FIG. 5), and second foils706 b″ that are disposed between the first foils 706 a″. In the presentembodiment, an example in which the bump foil consists of two firstfoils and two second foils will be described. A trailing edge F is aportion that connects the first foils 706 a″ to the second foils 706 b″in the end of the bump foil in the direction of rotation of the rotor.

As in the above embodiment, the bump foil 706″ may individually adjustthe stiffness of the first and second foils 706 a″ and 706 b″ byadjusting the widths (gaps) of slits 706 c″ and 706 d″ and the bumpwidths B to B″ of the bump foil 706″. In the present embodiment, it ispossible to adjust the stiffness of the first and second foils byadjusting the bump widths B to B″ of the second foils 706 b″ in detail.

In more detail, the bump foil 706″ is divided by a plurality of slits706 c″ and 706 d″, and a first slit 706 c″ between each of the firstfoils 706 a″ and the associated second foil 706 b″ has a shape differentfrom a second slit 706 d″ between the second foils 706 b″.

The first slit 706 c″ located between each of the first foils 706 a″ andthe associated second foil 706 b″ has the same width in the direction ofrotation of the rotor. That is, the first slit 706 c″ has a uniformwidth from one end between the first foil 706 a″ and the second foil 706b″ to an end toward the trailing edge F. In addition, the first slit 706c″ is closed at the end of the trailing edge F and is open at an endopposite to the end of the trailing edge.

On the other hand, the second slit 706 d″ located between two secondfoils 706 b″ has a different width from one end thereof to an end towardthe trailing edge F (the second slit is also open at the an end oppositeto the end of the trailing edge).

The second foils 706 b″ consist of a foil having a long length in thedirection of rotation of the rotor and a foil having a short length. Thesecond slit 706 d″ located therebetween has a stepped shape, and thewidth of the second slit 706 d″ is narrowed toward the trailing edge F.That is, the second foils 706 b″ may have different bump widths B to B″from one end thereof to the other end thereof for each section.

Accordingly, the second foil 706 b″ having a relatively long length hasa first stepped portion 7060 that is formed at one end thereof adjacentto the trailing edge F, a first connection portion 7061 that extends ina direction opposite to the trailing edge F from the first steppedportion 7060, a second stepped portion 7062 that is formed after thefirst connection portion 7061 extends by a predetermined length, and asecond connection portion 7063 that extends toward an opposite end ofthe trailing edge F from the second stepped portion 7062. Since thestepped portions are directed in the outer diameter direction, the bumpwidth B′ of the first connection portion 7061 is larger than the bumpwidth B of the second connection portion 7063.

The second foil 706 b″ having a relatively short length may have a thirdstepped portion 7065 that is formed at one side thereof facing the longsecond foil 706 b″ and is further spaced apart from the trailing edge Fthan the first stepped portion 7060. That is, the short second foil 706b″ may have a third connection portion 7064 that is formed adjacent tothe trailing edge F, and a fourth connection portion 7066 that is formedat an opposite side of the trailing edge F, and the third steppedportion 7065 may connect the third connection portion to the fourthconnection portion. In this case, since the third stepped portion isdirected in the inner diameter direction, the bump width (not shown) ofthe third connection portion 7064 is smaller than the bump width B″ ofthe fourth connection portion 7066.

The bump width B of the second connection portion 7063 and the bumpwidth B″ of the fourth connection portion 7066 may be equal to ordifferent from each other according to the specification of design.

The end of each of the first foils 706 a″, which is located opposite tothe trailing edge F, is formed by spot welding. The height of the firstfoil 706 a″ (the highest bump pitch height of the bump foil in crosssection) may be lowered by up to 50% compared to the second foils 706b″. Thus, the central region (region E) including the second foils mayhave a relatively lower stiffness than the first foils 706 b″. In moredetail, the welded first foils 706 a″ may have stiffness twice as muchas the non-welded second foils 706 b″.

In addition, it is possible to adjust the stiffness of the trailing edgeF portion by regulating the distance D from the ends of the slits 706 c″and 706 d″ to the trailing edge F such that the trailing edge F portionhas a higher stiffness than the central region (region E). Preferably,the distance D from the ends of the slits 706 c″ and 706 d″ to thetrailing edge F is designed to be less than or equal to twice the bumppitch C (D≤2C).

In the air foil bearing of the fourth embodiment, it is possible tochange the height of the bump ridge and the bump width in order toenhance a load support capability and a damping effect. A detailedstructure thereof will be described with reference to the drawings.Since the bump foil of the present embodiment is de designated byreference numeral different from but have the same structure as that ofthe fourth embodiments, a detailed description thereof will be omitted.

FIG. 10 is a top view illustrating an air foil bearing according to afifth embodiment of the present invention. FIG. 11 is a top viewillustrating an air foil bearing according to a sixth embodiment of thepresent invention.

As illustrated in FIG. 10, the air foil bearing has a structure in whicha bump foil 706 is inserted between a top foil 704 and a plate 702 andone end of the top foil 704 is a fixed end while the other end as a freeend of the top foil 704 is spaced apart from the plate 702. Accordingly,the top foil 704 has an inclined section L1 in which the gap between thetop foil 704 and the plate 702 is gradually increased from the fixed endof the top foil 704 to the free end thereof. The top foil 704 has a flatsection L2 in which the gap between the top foil 704 and the plate 702is constant at a predetermined position passing through the inclinedsection L1.

The flat section L2 is formed in a portion corresponding to the leadingridge A of the bump foil 706 in the direction of rotation of a rotor.The leading ridge A of each of first to fourth foils 706 a to 706 d is astarting position of the flat section L2.

When the leading ridge A of each of the first to fourth foils 706 a to706 d is set to have a lower height than an adjacent bump ridge (secondor more bump ridges) to form a gap between the leading ridge A and thehighest point, a thin oil film is first formed according to the shape ofthe top foil 704 due to the film stiffness of the top foil 704 and thestiffness of the second or more hump ridges.

When the RPM or load of a rotor 610 is increased, the load applied tothe bump foil 706 is gradually increased, thereby increasing thepressure of the oil film. In this case, when a thrust bearing 700 isdeformed over a certain level, the lowered leading ridge A is deformedby contact with the top foil 704 to support a load (activation). Inorder for the leading ridge A to be uniformly pressed when it isactivated, the leading ridge A is positioned in the flat section L2 ofthe top foil 704. Since the flat section L2 is increased while theleading ridge A supports a load, the thrust bearing 700 has a high loadsupport capability.

To this end, the leading ridge A preferably has a height lowered by upto 30% compared to other bump ridges. That is, the leading ridge A has aheight of 70% or less of the heights of other bump ridges.

When an air compressor 10 generates continuous vibration or shortintermittent vibration in a vertical or horizontal direction to set theheight of the leading ridge A, the rotor 610 is excited while vibratingby an external force. The rotor 610 has a set opening (a gap in aninitial position) with respect to the thrust bearing 700 in an initialposition, but the set opening is increased by excitation.

That is, the bump foil 706 is repeatedly pressed and restored. Inconnection with the displacement when the bump foil 706 is maximallypressed and restored, it can be seen that the bump foil 706 is pressedand restored by approximately 30% of the bump ridge of the bump foil 706(measurement of displacement of the rotor over time).

Accordingly, when a pressure is applied to the thrust bearing 700 over acertain level by setting the height of the leading ridge A within 30% ofthe heights of other bump ridges, it is possible to suitably support theload corresponding to the pressure.

Alternatively, it is possible to change the lengths of second and thirdfoils as in FIG. 11.

That is, the non-welded ends E of second and third foils 706 b′ and 706c′ may be positioned on the same line as the imaginary line connectingthe welded ends B of first and fourth foils 706 a′ and 706 d′.

The air foil bearing according to the embodiments of the presentinvention having the above configuration can enhance a load supportcapability and increase a damping effect by changing the shape andstiffness of the bump foil. In addition, it is possible to enhance thedurability of the bearing by an improvement in damping capability.

As is apparent from the above description, an air foil bearing accordingto exemplary embodiments of the present invention can adjust stiffnessof single bumps by varying widths of the single bumps. In addition,since the bumps have the same height in an initial stage, a wedgingeffect is implemented only when a load is applied thereto by an oilfilm. Therefore, the air foil bearing is advantageous in terms of heightmanagement thereof.

Furthermore, it is possible to stably dampen shocks applied in thedirection of rotation of a rotor, to improve a support capability, andto reduce shocks due to damping. Therefore, it is possible to enhancethe durability of the air foil bearing.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

What is claimed is:
 1. An air foil bearing for rotatably supporting arotary shaft, comprising: a disc-shaped plate, a plurality of bump foilscoupled to the plate, and a top foil disposed above each of the bumpfoils while one end of the top foil is coupled to the plate and theother end thereof is a free end, wherein each of the bump foilscomprises first to fourth foils extending from the coupled ends in adirection opposite to a direction of rotation of a rotor, wherein endsof the first to fourth foils are connected as one piece, and wherein aleading ridge (A) of each of the first to fourth foils, which isadjacent to the free end, has a height different from another adjacentbump ridge.
 2. The air foil bearing according to claim 1, wherein theheight of the leading ridge (A) of each of the first to fourth foils islower than the height of another adjacent bump ridge in the direction ofrotation of a rotor.
 3. The air foil bearing according to claim 2,wherein the height of the leading ridge (A) of each of the first tofourth foils is set to be within 70% of the height of another adjacentbump ridge in the direction of rotation of a rotor.
 4. The air foilbearing according to claim 3, wherein the top foil has an inclinedsection (L1) in which a gap between the top foil and the plate isgradually increased, and a flat section (L2) in which the gaptherebetween is constant.
 5. The air foil bearing according to claim 4,wherein the leading ridge (A) of each of the first to fourth foilscorresponds to a starting position of the flat section (L2) of the topfoil.
 6. The air foil bearing according to claim 3, wherein an end (B)adjacent to the leading ridge (A) of each of the first to fourth foilsis coupled to the plate by spot welding.
 7. The air foil bearingaccording to claim 1, wherein the bump foil is divided into first tofourth foils by a plurality of slits.
 8. The air foil bearing accordingto claim 7, wherein stiffness of the first to fourth foils is adjustedaccording to a width of each of the slits and a bump width of the bumpfoil.
 9. The air foil bearing according to claim 8, wherein some of thefirst to fourth foils have different bump widths from one end thereof tothe other end thereof.
 10. The air foil bearing according to claim 7,wherein the plurality of slits have different shapes.
 11. The air foilbearing according to claim 7, wherein the bump foil further comprises atrailing edge connecting the first foil to the second foil.
 12. An airfoil bearing for rotatably supporting a rotary shaft, comprising: adisc-shaped plate, a plurality of bump foils coupled to the plate, and atop foil disposed above each of the bump foils while one end of the topfoil is coupled to the plate and the other end thereof is a free end,wherein each of the bump foils comprises first to fourth foils extendingfrom the coupled ends in a direction opposite to a direction of rotationof a rotor wherein the bump foil is divided into first to fourth foilsby a plurality of slits, wherein the bump foil further comprises atrailing edge connecting the first foil to the second foil, and whereina distance (D) from an end of each of the slits to the trailing edge (F)is less than or equal to twice a bump pitch (C).